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Implements an FBSession subclass that knows about test users for a particular
application. This should never be used from a real application, but may be useful
for writing unit tests, etc.

Discussion:

Facebook allows developers to create test accounts for testing their applications'
Facebook integration (see https://developers.facebook.com/docs/test_users/). This class
simplifies use of these accounts for writing unit tests. It is not designed for use in
production application code.

The main use case for this class is using sessionForUnitTestingWithPermissions:mode:
to create a session for a test user. Two modes are supported. In "shared" mode, an attempt
is made to find an existing test user that has the required permissions and, if it is not
currently in use by another FBTestSession, just use that user. If no such user is available,
a new one is created with the required permissions. In "private" mode, designed for
scenarios which require a new user in a known clean state, a new test user will always be
created, and it will be automatically deleted when the FBTestSession is closed.

Note that the shared test user functionality depends on a naming convention for the test users.
It is important that any testing of functionality which will mutate the permissions for a
test user NOT use a shared test user, or this scheme will break down. If a shared test user
seems to be in an invalid state, it can be deleted manually via the Web interface at
https://developers.facebook.com/apps/APP_ID/permissions?role=test+users.

This method creates a session object which creates a test user on open, and destroys the user on
close; This method should not be used in application code -- but is useful for creating unit tests
that use the Facebook SDK.

This method creates a session object which uses a shared test user with the right permissions,
creating one if necessary on open (but not deleting it on close, so it can be re-used in later
tests). Calling this method multiple times may return sessions with the same user. If this is not
desired, use the variant sessionWithSharedUserWithPermissions:uniqueUserTag:.

This method should not be used in application code -- but is useful for creating unit tests
that use the Facebook SDK.

Array of strings naming permissions to authorize; nil indicates
a common default set of permissions should be used for unit testing

uniqueUserTag

A string which will be used to make this user unique among other
users with the same permissions. Useful for tests which require two or more users to interact
with each other, and which therefore must have sessions associated with different users. For
this case, consider using kSecondTestUserTag and kThirdTestUserTag so these users can be shared
with other, similar, tests.

+ (instancetype)

sessionWithSharedUserWithPermissions:

(NSArray *)permissions

uniqueUserTag:

(NSString *)uniqueUserTag;

Discussion:

This method creates a session object which uses a shared test user with the right permissions,
creating one if necessary on open (but not deleting it on close, so it can be re-used in later
tests).

This method should not be used in application code -- but is useful for creating unit tests
that use the Facebook SDK.